In an optical communications network, the wavelength of electromagnetic radiation used to communicate a signal is an important parameter. In particular, where Wavelength Division Multiplexing (WDM) systems are employed, different signals are respectively communicated using different wavelengths, each different wavelength being known as a channel. Typically, an array of semiconductor laser devices will be employed in a transmitter unit of the WDM system in order to transmit electromagnetic signals over a range of different wavelengths. In addition, or alternatively, a tunable semiconductor laser device capable of transmitting electromagnetic signals over a range of different wavelengths can be employed in the transmitter unit. Each channel corresponding to each laser device (or the tunable laser device) is separated from an adjacent channel by a set wavelength separation. For example, on an ITU grid, the wavelength separation between adjacent channels is 0.8 nm, which corresponds to a frequency separation of 100 GHz.
In WDM systems, the different electromagnetic signals on different channels are combined into a single transmission stream by a multiplexer which enables the different electromagnetic signals on the different channels of respective different wavelengths to be transmitted through a single optical fibre simultaneously. A demultiplexer is then used to separate out the different wavelengths corresponding to the electromagnetic signals onto individual optical fibres, each optical fibre consequently carrying electromagnetic radiation of a differing wavelength corresponding to a particular channel.
Laser devices employed in transmitters in a WDM system can exhibit wavelength drift as a result of a change in temperature of the laser device. Wavelength drift may result in a laser device transmitting an electromagnetic signal at a predominantly incorrect wavelength and therefore cause adjacent wavelength channels to overlap. The overlapping of adjacent wavelength channels is known as cross-talk and causes problems in the accurate multiplexing and demultiplexing of the electromagnetic signals in the WDM system. In order to monitor and control the wavelength drift of a laser device it is known to employ a wavelength dependant element in a closed feedback circuit. The wavelength dependant element can employ interference or dispersion to provide wavelength dependence. In relation to the use of interference, one known apparatus employing a Fabry-Perot etalon in a closed feedback circuit is disclosed in U.S. Pat. No. 5,825,792.